This invention relates to the separation of refractory metals such as chromium, molybdenum, vanadium, tungsten, niobium and tantalum from base metals such as nickel, cobalt, copper, and iron and to convert the refractory metals to a water-soluble form suitable for subsequent hydrometallurgical recovery.
The so-called superalloys provide strength and oxidation and corrosion resistance for relatively long periods of time at elevated temperatures, and are currently used in the fabrication of process equipment, steam and gas turbines and the like. They comprise a class of iron, nickel, and cobalt based materials alloyed with chromium, tungsten, molybdenum, niobium, and/or tantalum. Some superalloys also include relatively small percentages of aluminum and silicon.
Currently, a significant volume of superalloy scrap is produced in the United States in the form of fines, dust, grindings, and metal turnings. Because of the high value of the metals in the alloys, various processes have been developed which seek to provide an economical means of recovering metal products in commercially useful form from the scrap. However, because of the oxidation resistant and corrosion resistant nature of the so called super-alloys, recycling of this material poses many problems. Thus, no commercial process for such recycling is trouble free.
Furthermore, since these metallic wastes are often mixed with and are highly contaminated with either cutting oils and/or grinding media such as aluminum oxide and silicon carbide, they are unsuitable for direct remelting.
As is stated above, various processes have been developed which seek to provide an economical means of recovering commercially useful products from the scrap. One such process is disclosed in U.S. Pat. No. 3,649,487 to Aue et al. The Aue et al process involves pretreatment of the metal scrap with a group III, IV or V non-metal to tie up the molybdenum, tungsten, and chromium values as carbides, borides, silicides, nitrides, and/or phosphides. Anodes cast from this secondary material are placed in an electrolytic bath and are subjected to an electrolytic dissolution at low voltages such that the refractory metals are not dissolved or decomposed, whereby the nickel, cobalt, iron, and copper (if present) content may be recovered.
The foregoing process suffers from the disadvantage that the alloys must be remelted to produce the secondary material. Thus, the input of large amounts of energy is required and is expensive and, is prohibitively so for superalloy scrap relatively high in Fe content and low in Ni and Co content. Furthermore, carbiding has proved only marginally effective in inhibiting Cr dissolution in the electrochemical dissolution process. Consequently, additional operations are required to remove this contaminant from the electrolyte prior to the electrowinning of the base metals.
Various hydrometallurgical processes for recovering refractory metals are known in the art, but these are complicated by the presence of relatively large quantities of iron, nickel, cobalt, copper and mixtures thereof.
For example, U.S. Pat. No. 3,607,236 to P. T. Brooks discloses a method of reclaiming metal values from superalloy scrap. The Brooks process involves relatively unselective dissolution of the scrap in a chlorinated acidic aqueous solution and subsequent removal of tungsten, molybdenum, iron, cobalt, chromium and nickel from pregnant leach liquors by various extraction techniques.
If an energy efficient method of separating chromium, molybdenum, tungsten, vanadium, niobium, tantalum, and mixtures thereof from materials containing one or more metals such as nickel, cobalt, copper and iron were forthcoming, conventional hydrometallurgical recovery techniques could be utilized to greater advantage.